1. Field of the Invention
The present invention relates to the automatic drills of the type having feed and retract mechanisms for feeding a drill bit into a workpiece and for removing the drill bit from the resulting hole.
The predominant location for fatigue cracks in airframe structures are fastener holes. According to the present invention, our investigation has shown that the main cause of fatigue cracking in fastener holes are axial scratches produced upon retraction of the drill bit without simultaneous rotation. In many instances, we have foung that these fatigue critical scratches are not visible to the naked eye and are visible only when optically magnified. The axial scratches also occur in instances where the ratio of linear velocity relative to the surface rotational velocity of the drill bit during retraction is high.
2. Description of the Prior Art
One type of drill that has produced the undesirable axial scratches is the Winslow Spacematic Drill, models HS1 and HS2. This drill is a hand held air operated drill that produces close tolerance holes at speeds varing from 3600 to 8000 r.p.m. In its prior manner of operation, the air motors were turned off upon reaching a preset position during the feed stroke and the drill bit retracted immediately. This manner of operation during the retract mode has resulted in severe damage to the wall of the fastener hole. Damage in order of severity to fatigue behavior are abundant axial scratches, gouges, fluting marks, and surface roughness and waviness. The damage appears to be induced by the drill bit itself or by the drill bit axially pulling metal chips, that are cold worked and thus harder than the parent material, through the hole during the retract mode, scarring the surface.
Another drill observed which has produced axial scratches during the retract mode is a drill of the type manufactured by Quackenbush. One such drill rotates during retraction, however, the rotational speed during retraction is about 660 r.p.m. and the linear retraction rate is even faster than that of the Winslow drill.